Fuel combustion control system



Aug. 22,. 1939. w

FUBLQCOBUSTIONX CONTROL SYSTEM Filed Jan. 29, 1936 2 Sheets-Sheet 1 L 1n m9 U 5 o m i a 2 7 n y w mm m7 Z119 H @1726 Aug. 22, 1939.

w. GILLE FUELCQHBUSTION CONTROL SYSTEM Filed Jan. 29, 1936 2 Sheets-Sheet 2 9 mmw mnm ems nm A J) WMTI BEN .Qm. 29:20.

VVi'ZZisHT Gt'ZZe VMUN Patented Aug. 22,1939

Willis H. Gille, St. Paul, Minn, assignor to Min- Y neapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application January 29, 1936, Serial 'No. 61,398

24 Claims.

My' invention. relates to an improved fuel combustion control system and more particularly to such a control system having means for auto matically igniting the fuel. In this type of control system it is common to provide a thermal safety switch which is operative to interrupt the operation of the system after a predetermined period of time if combustion has not taken place. It combustion does take place within the prede termined period of time, as is normally the case, the thermal safety switch is deenergized so as to be inoperative. Various meanshave been employed for checking the presence of combustion preparatory to deenergization of the thermal safety switch. One such means comprises an electrode which is located in the presence of the flame of the burner and which is so connected in the system as to provide a conductive path of relatively low resistance to ground upon the presence of a flame so as to change the grid bias on a discharge device controlling the thermal safety switch. Since otherwise the means for checking the combustion would permit the system to continue in operation when a condition simulating combustion is present, such as a low resistance path between the electrode and ground arising by reason of carbonizatiomit is desirable that some means be provided for preventing the fuel supplying and ignition means from being turned on in the event that such a condition should arise.

An object of the present invention is to prov vide .a system of the class previously described made for preventing the means controlling combustion producing conditions from being energized until the thermionic element of the space discharge device has become sufliciently heated.

A further object of this invention is to provide a fuel combustion control system having means for deenergizing the ignition means in which provision'is made for delaying the action of said ignition deenergizing means for a predetermined period of time after combustion has taken place, while permitting immediate reenergization of the ignition means in the event of flame failure.

A further object of the present invention is to provide a 'relay circuit controlled by a grid controlled space discharge device in which provision is made for delaying the action of the relay by the introduction of a condenser in the grid circuit of said device, and in which means is provided for disconnecting and discharging said condenser upon energization of said relay.

Further objects of the invention will be apparent from the accompanying specification, claims and drawings, of which:

Figure 1 is a schematic view of one form of my control system, and of which: v

Figure 2 is a schematic View of a modified form thereof.

Referring to Figure 1 of the drawings, a gas burner is designated by the reference numeral l I. The flow of gas to the burner is controlled bya valve it which is preferably an electrically operated valve of any suitable type. The operating means for this valve is designated by the reference numeral l3. A pilot burner I4 is provided which pilot burner is controlled by an electrically operated valve 15 of anysuitable construction. Means are provided for igniting the pilot flame. This means comprises a pair of electrodes l6 which are connected to the opposite terminals of a secondary I I of a step-up transformer I8 having in addition to said secondary a line voltage primary l8.

The operation of my control system is controlled by means of a thermostat 20. The ther- The thermal safety switch 28 is preferably of the form disclosed in the patent to F. S. Denison,

No. 1,958,081, issued May a, 1934. As shown schematically in the drawings, this switch comprises two switch arms 30 and ii, the flrstof which is rigidly mounted and the second Joi' which is movably mounted Switch arm 3| is held in engagement with switch. arm 30 through a bimetallic element 32 which is rigidly secured at its lower end to a block ,33. A heater 34 is located in proximity to the bimetallic element 32. As indicated by the. legend on the drawings, the bimetallic element 32 is adapted to be deflected toward the right upon being heated. The heater circuits while switch arm 34 furnishes suiiicient heat so that if it remains energized for a predetermined period of time it will heat the bimetallic element 32 sufiiciently to cause it to be deflected to the right from under switch arm 3| allowing the same to drop downwardly and cause separation of switch arms 38 and 3! thus breaking the circuit controlled thereby. When such separation has taken place, the switch arms 30 and Si cannot be returned to their original position without manual operation thereof. It will be seen that the thermal safety switch acts as a timing switch, opening the controlled circuit after a predetermined period of time if the heater 34 which constitutes timer energizing means remains energized.

The transformer 29 supplies power for the operation of my control system. This transformer is a combination step-up and step-down transformer and comprises a line voltage primary 35, a low' voltage secondary 36, high voltage secondaries 31 and 38 and a low voltage secondary 39.

Three relays are employed in my control system. A first relay 4| comprises a relay coil 42 operatively associated with four relay arms 43, 44, 45 and 48. These relay arms are biased towards the right and when the relay coil is deenergized the arms 43, 45 and 48 are in engagement with contacts 41, 48 and 49, respectively. When the relay coil is energized arms 43, 44, 45 and 48 are in engagement with contacts 58, Si, 52 and 53, respectively. Relay M is primarily employed to check the operation of the system before permitting the other relays to be brought into operation and after combustion has been established to bring about the desired running conditions. All of the relay switch arms except switch arm 48 are employed in the controlling 88 controls the load circuits which energize the burner apparatus.

A second relay 54 comprises a relay coil 55 associated with a relay arm 58 which is adapted to engage with either of two contacts 51 or 58. Switch arm 58 is similarly biased towards the right and when the coil is deenergized is inengagement with contact 58. When the coil is energized arm 58 is moved into engagement with contact 51. Relay 54 serves primarily to cause relay 4! to be deenergized after its initial checking operation and to system to a position wherein combustion producing conditions can be established.

A third relay 59 comprises a relay coil 88 which has associated therewith three switch arms iii, 62 and 53. These arms are adapted to be moved into engagement with contacts 84, 85 and 68 upon energization of said coil. Relay 58 functions to bring about the conditions necessary for combustion, the switch arm 83 functioning as a load switch. Relay coils 55 and 80 of relays 54 and 59, respectively, are energized through secondary 38, while relay coil 42 of relay 4! is a direct current relay and is located in the plate circuit of a vacuum tube 81.

The vacuum tube 81 is of the well-known duplex-diode-triode type which functions both as a rectifier to supply a source of direct current for grid biasing purposes and as an, amplifier. The tube comprises the usual plate '69, a grid 18, a cathode H, a cathode heater 12 and a pair of diodes 13. The cathode heater 12 is connected through conductors 14 with the secondary 38 of transformer 28. The secondary 31 is connected in a circuit including the cathode and the two diodes. The two diodes 13 are connected tobring the elements of the 'gether since it is not necessary in the present case to have full wave rectification. A junction of the two diodes is connected through conductor 15 to one terminal of the secondary 81. The other terminal of secondary 31 is connected through conductor 16 to a conductor 11 which in turn is connected to one terminal of a condenser 18. The other terminal of the condenser is connected through conductors 19 and 8| with the cathode 1i. Thus the condenser 18 has impressed across its terminals a pulsating direct voltage produced by the diode unit, the pulsations of which are filtered out by the condenser. It will be noted that the cathode ll is connected to the positive side of the condenser. The negative side thereof is connected through conductors 82, 83 and 84, a resistor 85 and conductor 88 with the grid 10. The tendency, accordingly, of the voltage produced through the rectifying action of the diode 13 and cathode 1| is to bias the grid 18 negatively.

A second vacuum tube 81 is employed which tube is a conventional two-electrode half-wave rectifier, having a filament cathode which is energized by a portion of secondary 39, as indicated in the drawings. The output of secondary 38 is rectified through this tube and is connected across the terminals of a filtering condenser 89, one end of secondary 38 being connected to one terminal of condenser 89 and the other terminal of the secondary 38 being connected to the other terminal of the condenser through the tube 81. The negative side of the condenser 89 is connected through conductor 98 with the positive side of condenser 18 which is connected through conductor 8i with the cathode H. The positive terminal of condenser 88 is, under certain conditions, connected through various relay arms and contacts, to be presently described, to the grid 10. Thus the tendency of the power supplied by secondary 38 and rectifier 81 is to raise the potential of the grid with respect to the cathode. It will thus be seen that two power supplies are provided, which are connected in series and one of which is connected so as to bias the grid negatively and the other of which, when connected to the grid, is connected so as to tend to reduce said bias.

When the temperature of the space to be controlled falls below the value it is desired to maintain, the bimetallic element 2i of thermostat 28 will be effective to cause contact arm 22 to move into engagement with contact 23. This will close the circuit to the primary 35 of transformer '28 causing energization ofthe secondaries of the transformer. Energization of secondary 31 as soon as the cathode of tube 81 becomes heated causes a voltage to be applied across the terminals of condenser 18 which, because of the connections previously described, causes a negative biasing voltage to be applied to the grid 18 of tube 81. Energization of secondary 38 causes a voltage to be applied across the terminals of condenser 89, as soon as the cathode of tube 8'? becomes heated. As previously pointed out, the

negative terminal of the condenser 88 is connected together with the positive terminal of condenser 18 to the cathode 1i. With the various relays in the position shown, the grid 18 is connected to the positive side of condenser 88 through the following circuit: from grid 18, through conductor 88, the right-hand portion of resistance 85, conductor 96, contact 58, switch arm 58, conductors 85, 94, 83 and 82 to the positive terminal of condenser 89. The rsult of this connection of grid 19 to the positive terminal of the condenser 89 across which is applied the rec tified output of secondary 39 is that the grid is subjected to a positive voltage which reduces the negative biasing action of the previous power supply sufficiently to cause a current to flow between plate 69 and cathode 1I through the following plate circuit: from the positive terminal of condenser 89, through conductors 92. and 93, relay coil 42, conductor 99, plate 69, cathode 1I, conductors BI and 99 to the negative side 01 condenser 99.

This flow of current through the plate circuit including relay coil 42 is effective to cause energization thereof and to cause relay arms 43, 44, 45 and 46 to move into engagement with contacts 59, 5I, 52, and 53, respectively. The movement of arm 43 into engagement with contact 59 establishes the following circuit through relay coil 55 of relay 54: from the lower terminal of secondary .39, through conductors I99, 9I, 92, 93, 94 and IN, contact 59, switch arm 43, conductor I92,

relay coil 55-and conductor I93, to the other terminal of secondary 39. The establishment of this energizing circuit for relay coil 55 causes the relay arm 56 to be moved into engagement with contact 51. Upon this taking place the following holding circuit is established for relay coil 55: from the lower terminal of secondary 39, through conductors I99, 9I, 92, 93, 94, relay arm 56, contact 51, conductor I39, relay coil 55 and conductor I93 to the other terminal of secondary 39. At the same time that relay arm 56 is moved into engagement with contact 51, it is moved out of engagement with contact 59. Since the previously traced connection between the grid 19 and the positive side of the two power supplies depended upon the engagement of switch arm 56 with contact 58 this separation of the switch arm from contact 58 interrupts this previously traced connection, so that the grid is now subjected only to the negative biasing voltage applied across condenser 18. The result isthat the grid potential is now below the cut-off point so that no plate current flows through relay coil 42 the result that relay M is deenergized.

Upon the relay coil 55 being energized and relay coil 42 being deenergized, the following energizing circuit is established through relay coil 69: from the lower terminal of secondary 39' through conductors I99, 9I, 92, 93, 94, and 95, relay arm 56, contact 51,- the conductors I39 and I92, switch arm 43, contact 41, conductors I95 and I95, heating element 34, conductors I91 and I98, relay coil 69 and conductors I99 and I93 to the other terminal of the secondary 39. The energization of relay coil 69 causes contact arms BI, 62 and 63 to be moved into engagement with contacts 64, 65 and 66, respectively. The moving into engagement of contact arm 6I with contact 64 results in the following holding circuit being established for the relay coil 69: from the lower terminal of secondary 39, through conductors I99, 9|, 92, 93, 94 and 95, contact arm 56, contact 51, conductor I3I, contact arm 6|, contact 64, conductors I32 and I96, heating element 34, conductors I91 and I98, relay coil 69 and conductors I99 and I93 to the other terminal of secondary 39. The establishment of this holding circuit results in the energization of relay coil 69 being independent of relay coil 4I so that upon relay coil 4| becoming reenergized as subsequently takes place, relay coil 69 will remain energized. It will be noted that the energizing and holding circuits for relay coil 69 pass through the heating element with 34 of the thermal safety switch so that as soon as relay coil 69 is energized the heating element of the thermal safety switch'begins to heat the bimetallic element 32 thereof.

The moving into engagement of arm 63 with contact 66 results in the following circuit through the valve I5 controlling the pilot burner being established: from line wire 26, through conductors 25 and H9, load contact 66, load switch arm 63, conductor III, conductor H2, the pilot valve actuating means, and conductors H3, H4 and H5, to the other line wire 21. The establishment of this circuit causes the valve controlling the pilot to be opened with the result that gas is fed thereto. At the same time, the following circuit through the ignition means is established: from line wire 26, through conductors 25 and I I9, load contact 66, load switch arm 63, conductor III,

load switch arm 46, load contact 49, conductor II6, primary I9 of transformer I8 and through conductor H5 to the other line wire 21. The establishment of this circuit causes the energization of secondary I1 with the result that a high voltage is applied across the electrode I6,

causing a spark to appear thereacross.

With the energization of the ignition means and the opening of the pilot valve, combustion should take place if conditions are proper. My control system now functions so that after a predetermined period of time after a flame has appeared at the pilot, the main gas valve will be turned on and the ignition means turned oil. This is done by reenergizing relay 4I through reducing the grid bias by means of a connection between the grid and the positive side of the power' supplies through the flame. The action of the means for turning on the main gas valve is delayed through a condenser connected in parallel' with a portion of the grid circuit. At the same time, a portion of the resistance is cut out of the circuit between the grid and the negative terminal of the power supplies. This causes the grid to be lowered in potential so as to necessitate the presence of'a path of lower resistance than previously required between the gr'd and the positive side of the two power supplies in order to drive the grid voltage sufficiently high tocause energization of relay coil 42 These various means will now be described in more detail.

The means for connecting the grid to the positive side of the power supplies comprises an electrode I I8 which is inserted in the ath of the pilot flame. This electrode is connected through a conductor 9, a protective'resistance I29, conductor IZI and conductor 86 with the grid 19. The pilot supply pipe I4 is connected toground as indicated at I22 and the positive side of the two power supplies is connected to ground as indicated at I23. The result is that upon a flame of lower resistance than the air gap appearing at the pilot by reason of the same being lighted, a path between the grid 19 and the positive side of the positive power supply is established. The resistor I 29 is of comparatively low value and is inserted to prevent the grid from being subjected to too high a voltage in the event of. electrode I I8 being accidentally grounded.

grid and the negative terminal of the negative power supply in parallel to the connection through resistor 85. Until the condenser becomes charged this parallel connection offers a comparatively low impedance so that the grid tends to be directly connected to the negative terminal of the power supplies. After a predetermined period of time, depending upon the capacity of condenser I24 with respect to the resistance of the rest of the grid circuit the condenser will become charged and the potential of grid I will be accordingly raised. With the provision of this condenser, the effect of the connection between the grid and the positive terminal of the power supply through the fiame of the pilot burner is delayed until the condenser has been charged. ,It will be obvious that by choosing a condenser of suitable capacity, any desired time delay can be obtained. In this way, the action of ignition cut-off means can be delayed until there is a certainty that the flame is steady.

Until relay 59 is energized the connection between the grid I0 and the negative side of the two power supplies includes the entire resistance 85. As soon as relay 53 is energized, however, with the resultant moving into engagement of switch arm 62 with contact 65, the left-hand portion of resistance 85 is shunted out through the following circuit: from the left-hand terminal of resistance 85, through conductors 84, 83, I26 and I21, switch arm 62, contact 85, and conductors I35 and 06, to the mid-point 91 of resistance '85. The result of this is that until relay 59 is energized, the resistance between the grid and the negative side of the power supplies is much greater than after energization of said relay so that during the initial stages of operation of my system the grid is biased less negatively. The result of this is that if through insulation leakage or due to a low resistance between the electrode H8 and the pilot burner I4, there is a path of comparatively low resistance between the grid and the positive side of the two power supplies, which side is the grounded side, the grid bias will remain sufliciently low so that current continues to flow through relay coil 42. Since it is necessary for relay coil 42 to be deenergized before the relay coil 60 of relay 59 can be energized it is assured that relay coil 60 will not be energized in the event of excessive insulation leakage. As soon as the left-hand portion of the resistance 85 is shunted out, however, by reason of the en-ergization of relay 59, the bias of the grid 10 is increased so that a very substantial decrease in resistance between the grid and the positive side of the system is required in order to again reduce the grid bias sufficiently to cause reenergization of relay coil 42. In this way, a check is made on the insulation resistance of the system while at the same time provisionis made against a faulty check of the presence of a flame due to the grid bias being so high that the system is too sensitive. By the shunting out of a portion of the resistance 85 it is necessary that there'be a material reduction in the-resistance in the connection between the grid and the positive side of the two power supplies such as would occur only in the event of a flame bridging the gap between the pilot and the electrode II8. Moreoventhis insures a recycling in the event of the pilot flame being extinguished since even though the electrode has become carbonized in use, the resistance between the electrode and ground will be too high to maintain the bias sufllciently low to permit a flow of plate current through relay 4I.

It will be noted that the connection between the grid and the positive side of the power supplies 'through relay 54 includes the right-hand portion of resistance 85. As is well known, it is extremely diiilcult to adequately insulate members of a relay without impeding its proper functioning. Since the high potential side of the system is grounded, it is very important that there is no leakage path of comparatively low resistance between the grid and ground. By connecting the grid to relay contact 58 through the right-hand portion of resistance 85, which has a rather large value, there is an assurance that any path between the grid and the grounded high potential side of the system will be of such high resistance as to not appreciably raise the potential of the grid.

If conditions are proper and a flame does appear, as will normally be the case, relay 4| will be energized with the result that arm 46 is moved out of engagement with contact 49 and into engagement with contact 53. The moving of arm 46 out of engagement with contact 49 causes the previously traced circuit through the primary of the ignition transformer to be interrupted with the result that the ignition means is now deenergized. At the same time, the moving into engagement of load switch arm 46 with contact 53 results in the following circuit being established to the main gas valve: from line wire 26, through conductor 25, conductor IIO, load contact 66, load switch arm 83, conductor III, load switch arm 46, load contact 53, conductor I29, the valve actuating means I3, conductors I30, H4 and M5 to the other line wire 21. This causes the main gas valve to be opened with the result that gas is admitted to the burner which gas is lighted by the pilot flame. I

The reenergization of relay H with the resultant moving into engagement of contact arm 44 with contact 5I results in the following new holding circuit for relay coil 60 being established: from one end of the secondary 39, through conductors I00, SI. 92, 93, 04, 95, contact arm 56, contact 51, conductor I3I, switch arm 6|, contact 64, conductors I32, I05 and I33, switch arm 44, contact 5|, conductor I34, conductor I08, relay coil 60 and conductors I08 and I03 to the other terminal of secondary 39, It will be noted that the new holding circuit for relay coil 80 just traced does not include the resistance 34 of the thermal safety switch 28 but in efiect constitutes a shunt thereacross. Due to the rather high resistance of resistance 34 very little current will flow therethrough and bimetallic element 32 will not be appreciably heated thereby.

The reenergization of relay 4I also causes the engagement of arm 45 with contact 52 with the result that the condenser I24 is now connected in a circuit including conductors I40 and I28, contact 52, contact arm 45, and conductor I25. The condenser I24 will discharge immediately due to the absence of appreciable resistance in the circuit so that in the event the flame is extinguished for any reason, it is possible to immediately reenergize the ignition means, even though such a flame failure takes place almost immediately after the deenergization of the ignition means.

Iffor any reason the pilot burner does not become lighted, the potential of grid 10 will not be with the result that the current flowing therethrough must pass through the heater 34. After a predetermined period of time this will cause the bimetallic element 32 to be heated sufliciently to result in a separation of switch arms 30 and 3|. The separation of the switch arms opens the circuit to the primary of transformer 29 and deenergizes the entire system. As previously stated, the switch arm 28 cannot be automatically reset so that it is necessary for the attendant to investigate the condition which prevents ignition of the pilot burner before the system can again be put into operation.

Moreover, since it is necessary for the relay 4| to be energized and deenergized before energization of relay 59, which controls the ignitionv for the pilot fuel supply, is possible, there is an assurance that there will be no faulty check of the presence of combustion due to the presence of a low resistance path between the electrode I I8 and the pilot burner I4. If such a condition should exist the relay 42 would not be deenergized so that relay 59 would never be energized and the fuel supply and. the ignition means would not be brought into operation. Moreover, there is an assurancethat there will be no delay in checking the presence of a flame preparatory to shunting of the thermal safety switch by reason of the time required to heat the filaments of tubes 61 and 81. Before relay 59 can be energized it is necessary for relay 42 to be energized and this cannot take place until the filaments are heated. By thus requiring energization and deenergization of the relay 42 which ultimately is energized to shunt out the thermal safety switch I guard in a positive manner against a condition which might arise through a shorted electrode wherein the thermal safety switch would be shunted out even though there was no combustion with the result that the ignition and fuel supply means would continue in operation without combustion of the fuel being supplied taking place.

If at any time both the pilot burner and the main burner are extinguished the conductive path between the grid and the positive terminal of the two power supplies will be. interrupted with the result that relay coil 42 is deenergized. This will cause immediate reenergization of the ignition means and closing of the main gas valve and at the same time interrupt the last traced holding circuit of relay coil 60. If conditions are proper to reestablish combustion the presence of the flame will cause reenergization of relay 4| as soon as condenser I24 is recharged. On the other hand, if for some reason combustion does not take place the continued flow of current through heater element 34 will open the thermal safety switch and deenergize the entire system.

It will be noted that I have provided means for thoroughly testing the operat on of my flame responsive ignition deenerglzing circuit before establishment of combustion producing conditions. Thus, if for any reason the tube 61 is not functioning in the manner desired, relay coil 42 will never be energized. If, on the other hand, the tube 61 should be functioning properly but the resistance across the flame gap should be so low that the flame checking means might otherwise work even in the absence of a flame, the relay coil 42 is not initially deenergized so thatit is impossible for the power relay 60 to be energized so as toestablish combustion under these conditions. Again, if the rectifier tube 81 is not functioning properly, relay coil 42 will not be energized. At the same time I avoid the disadvantage of certain prior devices wherein slight varia tions in the conductivity of the flame will cause the system to be deenergized. It will be noted that no matter how low the conductivity of the flame, the system will remain in operation. While it is true that a too high resistance across the flame gap will cause deenergization of relay 4| and if continued long enough deenergization of the entire system, a large variation in resistance is possible due to the fact that the portion of the resistance 85 between its midpoint and terminal 84 is shunted out and the flame checking means, is accordingly less sensitive than-in the testing period of its operation.

In Figure 2 I have shown a modified form of my combustion control system. In this form of my device I have shown my system as applied to an oil burner, although it will be understood that the same could readily be applied to any other fluid fuel burner by obvious modifications thereof.

Referring to said figure, a fuel burner is designated by the reference numeral I50 and comprises a burner motor |5I driven by a motor having a terminal box I52 and a fuel feeding nozzle I53 grounded as indicated at I54,

The means for igniting the fuel comprises a step-up transformer I55 comprising a line voltage primary I56 and a high voltage secondary I51. The opposite terminals of the secondary I51 are connected to electrodes I58 spaced apart to form a spark gap therebetween. A combination step-up, step-down transformer I59 supplies power for the operation of my control system. This transformer comprises a line voltage primary I60, low voltage secondaries I6I and I64 and two high voltage secondaries I62 and I63. One terminal of the primary I68 is connected to line wire I65 to a conductor I66. The other terminal is connected to line wire I61 through a thermal safety switch I68 and a thermostat I69.

The thermostat I69, which may be of any form, is shown as a conventional bimetallic thermostat having a bimetallic element I19 to which is attached a contact arm I'II adapted to engage with a contact I12,

The thermal safety switch, which is of the same type as a similar switch 28 in the previously described species, consists of a pair of switch arms I13 and I14 which are held in engagement by bimetallic element I15. An electric heater I16 is located in proximity to the bimetallic element. Since the operation of this switch is the same as switch 25 it will not be repeated here. 1

As in the previous form of my control system, three relays I11, I18 and I19 are employed. Relay I'II corresponds in function to relay 4| of the previously described species and comprises a relay coil IBD and switch arms I8I, I82, I83 and I84. In the deenergized position of relay coil I88, switch arms I81, I82, I83 and I84 are in engagement with contacts I85, I86, I81 and I88, respectively, Energization of relay coil I85 causes disengagement of the switch arms with the previously named contacts and causes switch arms I8I and I82 to be moved into engagement with contacts I59 and I 98, respectively. The switch arm I92 is connected through a one-way connection to the armature which cooperates with relay coil I88 so that upon deenergization of relay coil I88, arm I82 may still be held in engagement with contact I98 by other means without preventing the return movement of the other switch arms. As in the previous species, all of the switch arms except switch arm I84 are used in the various control circuits, arm I84 being used to control the load circuit.

The relay I18 comprises a relay coil I9I which is not of sufficient strength to cause switch arm I82; with which it is associated, to be moved into engagement with contact I90 but is able to hold the arm in engagement with said contact after the engagement has been established by relay coil I80. The arm I82 is also connected through a one-way connection to the armature of coil I9I so that the coil I80 may move blade I82 into engagement with contact 90 without moving the armature of coil I9I.

Relay I19, which constitutes the power relay, comprises a relay coil I92 which is associated with switch arms I93 and I94 and is adapted to move these arms into engagement with contacts I95 and I96 upon energization thereof. The switch arm I93 is employed to establish a holding circuit for the relay coil, while switch arm I94 acts to control the load circuit.

As in the preceding species two direct current power supplies are employed, one of which is obtained through the rectifying action between the diodes and the cathode of a duplex-diode-triode tube I91 and the other of which is obtained through a two-electrode rectifier I98. The duplex-diode-triode tube I91 is of the conventional type and comprises a cathode I99, a grid 200, a plate MI and the two diodes 202. The cathode I99 is of the indirectly heated type employing a heater 203 which is energized through the secondary I6I of transformer I59. The secondary I62 is employed to furnish the alternating current rectified by the diodes 202 and cathode I99. One terminal of the secondary I62 is connected to the two diodes 202 which are shown as connected in parallel since for the present purposes it is not necessary to have full wave rectification. The other terminal of the secondary I62 is connected to one terminal of a condenser 204, the opposite terminal of which is connected to the cathode I99. Thus, there is provided across the condenser 204, a direct pulsating voltage, the pulsations of which are filtered out by the condenser.

The secondary I63 is connected to the opposite terminals of a condenser 205 through a conductor 206 and a rectifier I98. The result is that a second supply of direct current is available across the terminals of condenser 205, the pulsations of which are filtered out by the condenser. The two power supplies are connected in series through a conductor 201 which connects the negative terminal of condenser 205 with the posi' tive terminal of condenser 204. The grid 200 is permanently connected to the negative side of the two power supplies through a resistance 200 which resistance corresponds to resistor 85 in my preferred species. As in my preferred species, a. further connection is provided between the grid and the positive side of the two power supplies which connection may be either through the switch'arm I82 or through the burner flame.

The secondary I64 is designed to supply current for the energization of filament 2I0 of the rectifier I98. Under certain conditions the secondary also supplies current to the heating element I16 of the thermal safety switch I68.

Upon the temperature in the room or.other space to be heated falling below the desired value the thermostat I69 is closed causing the energization of transformer I59. Upon this taking place, the cathode I99 of the tube I91 is heated. As soon as the cathode becomes sumciently heated, current is passed between the diodes 202 and the cathode and causes a direct voltage to be applied across the terminals of condenser 204. At the same time the filament 2 I0 of rectifier I96 becomes heated with the result that a voltage is impressed across the terminals of condenser 205.

Because of the previously described connections, the voltage applied across condenser 204 is efiective to bias the grid negatively with respect to the cathode. As previously indicated, the negative terminal of condenser 205 is connected together with the positive terminal of condenser 204 to the cathode I99. With the parts shown in the drawings, the grid 200 is connected with the positive side of the condenser 205, which is the positive side of the two power supplies, through the following circuit: from grid 200, through conductor 2I2, protective resistor 2I9, conductor 2I8, contact I86, contact arm I82, resistor 2I1, conductor 2I8, and conductor 2I5, to the positive terminal of condenser 205. The result of this connection between the grid 200 and the positive side of the two power supplies is that the bias of grid 200 is reduced sufliciently that current flows between the plate and cathode of tube I91 through the following plate circuit: from the upper terminal of secondary I68, through the rectifier I98, conductors 242 and 2I5, relay coil I08, conductor 222, plate 20I, cathode I99 and conductors 220, 201 and 206, back to the other terminal of secondary I63. time a circuit is established through relay coil I9I as follows: from one end of secondary I63 through the rectifier I98, conductors 242, H and 2I6, resistor 2I1, conductor 223, relay coil I9I, conductor 224, resistor 225, conductors 226, 220, 201 and 206 to the other terminal of secondary I63. It will be noted that the energization of relay coil I80 depends upon a flow of plate current through tube I91 while the energization of relay coil I9I is independent of tube I91.

It will further be noted that the circuits just traced included resistors 208, 2I1, 2I9 and 225.

The function of resistance 208 is to reduce the negative biasing eflect of the connection between the grid and the negative side of the power supplies, to a value desired. Resistors 2I1 and 226 are solely to reduce the voltage applied to relay coils I9I and I92 to a value for which they are designed. Resistor 2I9 functions as a protective resistance to prevent any danger of an excessive voltage being applied to grid 200.

Upon the energization of relay coil I80, switch arm I82 is moved'out of engagement with contact I86 and into engagement with contact I90. The movement of the switch arm I82 out of engagement with contact I86 results in the connection between the grid and the positive side of the two power supplies being interrupted so that the grid is connected only to the negative side of the two power supplies and accordingly is biased negatively to such an extent as to prevent flow of plate current through tube I91. This results in the deenergization of relay coil I80. In view, however, of the fact that relay coil I9I is independent of tube I91 and is able to hold switch arm I82 into engagement with contact ductor 228, relay coil I92, conductors 229, 226,

220, 201 and 206 back to the other terminal of secondary I63.

The energization of relay coil I92 causes switch arms I93 and I94 to be moved into engagement with contacts I95 and I96, respectively. The engagement of switch arm I93 with contact I95 establishes a holding circuit for relay coil I92 as follows: from one terminal of secondary I63, through the rectifier I98, conductors 242, 2I5 and 2I6, resistor 2I1, conductor 230, switch arm I93, contact I95, relay coil I92, and conductors 229, 226, 220, 201 and 206 back to the other terminal of secondary I63. It will be noted that this holding circuit is independent of the engagement of switch arm I83 with contact I81 so that even if relay coil I80 is now reenergized, relay coil I92 will still remain energized.

The engagement of switch arm I94 with contact I96 as a result of energization of relay coil I92 establishes the following circuit through the burner motor: from line wire I61, through conductors 23I and 232, switch arm I94, contact I96, conductor 233, the burner motor, conductors 250 and 234 to the other line wire I65. At the same time the following circuit is established through the primary of theignition transformer: from line wire I61, through conductors 23I and 232, switch arm I94, contact I96, conductor 235, contact I88, switch arm I84, conductor 236, primary I56, conductors 231 and 234, to the other line wire I65. With the energization of the primary I56, the ignition means is placed in operation and conditions are established for combustion.

If the apparatus is functioning properly, the fuel issuing from the nozzle I53 will be ignited by the spark appearing across the electrodes I58 and a flame will appear which will bridge 'the gap between an electrode 238 and a burner nozzle I53. Upon this taking place, a new path is established between the grid and the positive side of the two power supplies, through conductor 2I2, resistor 2I9, conductor 239, electrode 238, and the flame gap to burner nozzle I53 which is connected to ground so as to be at the same potential as the positive side of the power supply. The result of the establishment of this new connection between the grid and the positive side of the power supply is to again raise the grid to a potential sufliciently high to cause the tube I91 to pass current. The result of this is to cause relay coil I80 to be energized again. The energization of relay coil I80 causes separation of switch arm I 84 and contact I88 to interrupt the circuit through the ignition transformer to deenergize the ignition means. Since during this initial state, the flame is apt to be somewhat unsteady it is desirable, as pointedout in connection with my previous species, to somewhat delay the action of the relay which places the ignition means out of operation. In-my-previous species this was accomplished by the condenser connected across the portion of the grid circuit, which delayed building up of the potential of the grid. In the present case, a condenser 240 is connected across the relay coil I80 so as to effectively shunt out the relay coil I80 until the condenser 240 has become charged. The time required for charging is sufllcient to permit the flame to become somewhat stable.

As indicated in the previous paragraph, the

secondary I64 is employed to heat the filament 2I0 of rectifier I98 and also to supply-current to heating element I16 during certain portions of the operation of this system. As will be apparent from the drawings, so long as relay coil I80 is deenergized, the heating element I16 is being energized through a circuit as follows: from one terminal of secondary I64, through conductor 2, the filament 2I0, conductor 242, conductor 2I5, conductor 2I6, switch arm I8I, contact I85, conductor 243, heating element I16 and conductor 244 to the other terminal of secondary I64. Upon energization of relay coil I80, switch arm IBI is moved out of engagement with contact I85 and into engagement with contact I89. The circuit through the heater- 2I0 is now as follows: from one terminal of secondary I64 through conductor 24I, filament 2I0, conductors 242, 2I5 and 2I6, switch arm I8I, contact I89 and conductor 245 to an intermediate tap 246 of the secondary I64. It will be noted that under these conditions the heater element I16 is not receiving current and only a portion of the secondary I64 is connected in the circuit. The portion of the secondary I64 which is connected in the circuit under these conditions is of such value that the current flowing through the filament 2I0 will be of substantially the same value as when the entire secondary is connected in the circuit and the heating element I16 is included therein.

With the above described arrangements of the heating element of the thermal safety switch there are two occasions upon which the thermal safety switch is likely to be open. The first of these exists the time that the system is first energized before the relay coil I80 has become energized. It will be seen that at this stage of the operation if relay coil I80 does not become energized within a predetermined period of time for any reason whatsoever, the heater I16 will become sufliciently heated to open thethermal safety switch and deenergize the entire system. In this way the condition of the two tubes I91 and I98 and the various connections are checked before any of the relays have operated. A second occasion upon which the thermal safety switch is apt to be open is after relay I11 has first been deenergized with the resultant energization of relay I19 which establishes a combustion producing condition. If for any reason at its time combustion does not take place the grid 200 remains permanently at the lower potential so that relay coil I80 is not again reenergized. In this case also the heating element I16 will remain in the circuit sufficiently long to open the thermal safety switch.

While in the present species I have not provided a means for increasing the sensitivity of tube I91 to a reduction in the conductivity between the electrode 238 and the burner nozzle as was done in the preferred species by having a resistance of considerable magnitude which is connected between the grid andthe negative terminal of the two power supplies during the initial operation of the system and which is partly cut out after establishment of combustion, it will be understood that this means could be employed with the present species in the same manner as in the previously described species where it is considered desirable. Furthermore, while I have shown a condenser connected directly across the relay coil I80 instead of across a portion of the grid circuit of the duplex-diode-triode tube as in the previously described species, it will be understood that if desired the condenser may be so connected in the present series. Another difference between the last described species and the first described one is that the grid is not connected to the positive terminal of the power supplies through a portion of the resistor between the grid and the negative terminal of the power supplies as was done in the first described species. It is to be understood, however, that this feature may be employed in connection with the second species, if desired. Furthermore, while, in one case, my invention is applied to a gas combustion system and in another case to an oil combustion system, the invention is not limited to any particular type of fuel combustion system.

In addition to the above modifications, it will be understood that such other changes may be made as would occur to one skilled in the art and that my invention is limited only by the scope of the appended claims.

I claim as my invention:

1. In a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, means for terminating operation of said fuel igniting means, and means for delaying the action of said last named means for a predetermined period of time after combustion has first taken place, said delaying means being operative upon the operation of said fuel igniting means being terminated to immediately assume the condition that it had before said fuel feeding means and fuel igniting means were brought into operation.

2. In a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, means for terminating operation of said fuel igniting means, energy storing means for delaying the action of said last named means for a predetermined period of time after combustion has first taken place, and means for immediately removing the energy stored up in said energy storing means upon the operation 'lf said fuel igniting means being terminated.

3.1m a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, electrically operated means for terminating operation of said fuel igniting means, a source of electrical energy, a circuit in cluding a gap bridged by a flame when combustion takes place acting to connect said source of electrical energy with said last named electrlcally operated means, a. condenser, and connections between said condenser and a portion of said circuit operative to delay the action of said means for terminating the operation of said ignition means until said condenser has become charged.

4.111 a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, electrically operated means for terminating operation of said fuel igniting means, a source of electrical energy, a circuit including a gap bridged by a flame when combustion takes place acting to connect said source of electrical energy with said last named electrically operated means, a condenser, connections between said condenser and a portion of said circuit operative to delay the action of said means for terminating the operation of said ignition means until said condenser has become charged, and means operative upon said ignition means being rendered inoperative to immediately discharge said condenser.

Ea. In. a burner control system, fuel feeding amass? means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, means including an electrical circuit for terminating the operation of said fuel igniting means, a switch in said circuit and controlling the same, a relay coil controlling said switch, an energizing circuit for said relay coil and a condenser connected across said relay coil, said condenser being of sufficient capacity relative to the resistance of said circuit means to appreciably retard the energization of said relay upon establishment of said energizing circuit by reason of the time required to charge said condenser.

6. In a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, means including an electrical circuit for terminating the operation of said fuel igniting means, a switch in said circuit and controlling the same, a relay coil controlling said switch, a source of electrical energy, circuit means connecting said source of electrical energy to said relay coil, said circuit means including a grid controlled electric valve for controlling the flow of electrical energy from said source to said relay, a condenser, and connections between'said condenser and said grid effective to retard the flow of current to said relay coil until said condenser has been charged.

7. In a burner control system, fuel feedi means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, means including an electrical circuit for terminating the operation of said fuel igniting means, a. switch in said circuit and controlling the same, a relay coil controlling said switch, a source of electrical energy, circuit means connecting said source of electrical energy to said relay coil, said circuit means including a grid controlled electric valve for controlling the flow of electrical energy from said source to said relay, a condenser, connections between said condenser and said grid effective to retard the flow of current to said relay coil until said condenser has been charged, a second switch controlled by said relay coil, and means controlled by said second' switch operative upon energization of said relay coil to discharge said condenser.

8. In a burner control system, a burner, igniting means for said burner, a space discharge device comprising a cathode, a grid, a main anode, and an auxiliary anode, a relay coil, a normally closed switch controlled by said coil, an energizing circuit for said ignition means controlled by said switch, a source of alternating current, connections between said source of alternating current, said cathode, said auxiliary anode and said grid to maintain aconstant negative biasing voltage on said grid, rectifying means, connections between said source of alternating current, said rectifying means, said relay coil, said cathode and said main anode to maintain said main anode at a constant positive voltage with respect to said cathode, and further variable connections between said source of alternating current, said rectifying means, said cathode and said grid and including a flame gap adjacent said burner, to apply a positive biasing voltage to said grid upon the presence of a flame at said burner.

9. In a burner control system, a burner, electrically controlled fuel feeding means therefor, fuel igniting means, switching means for bringing into operation said fuel feeding means, means operative in the absence of combustion to render 76 inoperative said fuel feeding means a predetermined period of time after the same has been brought into operation, further switching means, said first named switching means being operative only after an initial predetermined operation of said further switching means including the movement of said switching means between two of its circuit controlling positions, and means for preventing the completion of said initial predetermined operation of said further switching means in the event of a condition simulating that of combustion.

10. In a burner control system, a burner, fuel feeding means therefor, fuel igniting means, switching means, movable between first and second circuit controlling positions, means for normally causing said switching means to move from said first to said second and back to said first controlling position, electrically operated means operative upon said switching means so moving from first to second and back to said first controlling position to bring into operation said fuel feeding means, means operative in the absence of combustion to render inoperative said fuel feeding means a predetermined period of time after the same has been brought into operation, and means for preventing said switching means from being moved back from said second to said first position in the event of conditions simulating those of combustion.

11. In a burner control system, a burner, electrically operated fuel feeding and fuel igniting means, a first relay coil, first and second relay switches closed upon energization of said relay coil, a third relay switch normally closed but opened upon energization of said relay, a second relay coil, a load switch closed upon energization of said relay coil, a source of energy, a timing switch controlling said source of energy, means for successively energizing and deenergizing said first relay coil, means operative to establish an energizing circuit for said second relay coil upon the successive closing of said first and third relay switches by reason of the successive energization and deenergization of said first relay coil, said last named energizing circuit including timer energizing means for said timer switch, an energizing circuit for said fuel feeding and said fuel igniting means controlled by said load switch, means responsive to combustion for reenerglzing said first relay coil, a shunt circuit around said timer energizing means controlled by said second relay switch to deenergize said timer energizing means as the result of combustion being established, and means operative in the event of conditions simulating combustion prior to the bringing into operation of said fuel feeding and fuel igniting means to prevent the initial deenergization of said first relay coil so as to prevent said fuel feeding and fuel igniting means being brought into operation.

12. In a burner control system, a burner, fuel flow controlling means, fuel igniting means, a first relay coil, a first load switch which is normally closed but is opened upon energization of said coil, a second relay coil, a second load switch which is closed upon energization of said second coil, energizing means, an energizing circuit for said fuel flow controlling means including said energizing means and controlled by said second loadswitch, an energizing circuit for said fuel igniting means including said energizing means and controlled by both said first and second load switches, means for energizing said second relay coil to establish the circuit through the fuel flow controlling means and the fuel igniting means, means operative upon the establishment of combustion to energize said first relay coil to interrupt the circuit to the fuel igniting means, and further means to energize and deenergize successively said first relay coil prior to the energization of said second relay coil in order to test the operation thereof prior to the cstablishment of combustion producing conditions.

13. In a burner control system, a burner, fuel flow controlling means, fuel igniting means, a first relay coil, a first load switch which is normally closed but is opened upon energization of said coil, a second relay coil, 9. second load switch which is closed upon energization of said second coil, energizing means, an energizing circuit for said fuel fiow controlling means including said energizing means and controlled by said second load switch, an energizing circuit for said fuel igniting means including said energizing means and controlled by both said first and second load switches, means for energizing said second relay coil to establish the'circuit through the fuel flow controlling means and the fuel igniting means, means operative upon the establishment of cornb ustion to energize said first relay coil to interrupt the circuit to the fuel igniting means, and further means including a third relay coil and a control switch to energize and deenergize successively said first relay coil prior to the energization of said second relay coil in order to test the operation thereof prior to the establishment of combustion producing conditions.

14. In a burner control system, a burner, fuel flow controlling means, fuel igniting means, a first relay coil, a first load switch which is normally closed but is opened upon energization of said coil, a second relay coil, a second load switch which is closed upon energization of said second coil, energizing means, an energizing circuit for said fuel flow controlling means including said energizing means and controlled by said second load switch, an energizing circuit for said fuel igniting means including said energizing means and controlled by both said first and second load switches, means for energizing said second relay coil to establish the circuit through the fuel flow controlling means and the fuel igniting means,

an energizing circuit for said first relay coil, a

said ignition means, and further connections in i said grid circuit operative before establishment of combustion producing conditions to raise the grid potential sufficiently to cause energization of said first relay.

15. In a burner control system, a burner, fuel fiow controlling means, fuel igniting means, a first relay coil, a first load switch which is normally closed but is opened upon energization of said coil, a second relay coil, a second load switch which is closed upon energization of said second coil, energizing means, an energizing circuit for said fuel flow controlling means including said energizing means and controlled by said second load switch, an energizing circuit for said fuel igniting means including said energizing means and controlled by both said first and second load switches, means for energizing said second relay coil to establish the circuit through the fuel fiow controlling means and the fuel'igniting s, an energizing circuit for said first relay coil, a grid controlled space discharge device controlling the flow of current through said circuit, a grid circuit for said tube including connections effective upon the presence of a fiame to raise the potential of the grid sufliciently to cause the space discharge device to pass sumcient current to energize said first relay coil so as to deenergize said ignition means, and further connections in said grid circuit operative before establishment of combustion producing conditions to raise the grid potential sufficlently to cause energization of said first relay, and further means operative upon energization of said first relay coil to interrupt said last named connections prior to establishment of combustion producing conditions.

16. In a burner control system, a burner, fuel fiow controlling means, fuel igniting means, a first relay coil, a first load switch which is normally closed but is opened upon energization of said coil, a second relay coil, a second load switch which is closed upon energization of said second coil, energizing means, an energizing circuit for said fuel flow controlling means including said energizing means and controlled by said second load switch, an energizing circuit for said fuel igniting means including said energizing means and controlled by both said first and second load switches, means for energizing said second relay coil to establish the circuit through the fuel fiow controlling means and the fuel igniting means, an energizing circuit for said first relay coil, a grid controlled space discharge device controlling the fiow of current through said circuit, a grid circuit for said tube including connections efiective upon the presence of a fiame to raise the potential of the grid sufficiently to cause the space discharge device to pass sufiicient current to energize said first relay coil so as to deenergize said ignition means, and further connections in said grid circuit operative before establishment of combustion producing conditions to raise the grid potential sumciently to cause energization of said first relay, and further means operative upon energization of said first relay coil to interrupt said last named connections to deenergize said first relay coil and to cause operation of the means for energizing said second relay coil.

17. In a fuel burner control system, a burner, fuel feeding means, fuel igniting means, means for establishing energizing circuits for said fuel feeding and said fuel igniting means to place them into operation, a space discharge device, connections to said device to provide input and output circuits, a relay in the output circuits including a. switch normally open but closed upon energization of said relay coil, said energizing circuit for said fuel igniting means being controlled by said switch so that energization of said relay coil is efiective to deenergize said igniting means, means operative upon the establishment of a flame adjacent said burner to vary the potential across the input circuit in a manner to cause a current flow in said output circuit suflicient to energize said relay, a condenser connected across a portion of said input circuit and adapted to delay said change in potential across said input circuit and consequently the deenergization of said igniting means until said condenser is charged, and means operative upon said relay being energized to disconnect said condenser from said input circuit and to discharge the same.

18. In a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, a cut-out mechanism automatically operable after a timed period to interrupt the operation of said fuel feeding means, means for rendering said cut-out mechanism inoperative, and means for delaying the action of said last named means for a predetermined period of time after combustion has first taken place, said delaying means being operative upon said cut-out mechanism being rendered inoperative to immediately assume the condition it had before said fuel feeding and fuel igniting means were brought into operation.

19. In a burner control system, fuel feeding means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, a cut-out mechanism automatically operable after a timed period to interrupt the operation of said fuel feeding means, means for rendering said cut-out mechanism inoperative, energy storing means for delaying the action of said last named means for a predetermined period of time after combustion has first taken place, and means for immediately removing the energy stored up in said energy storing means upon said cut-out mechanism being rendered inoperative.

20. In a burner control system, fuel feedin means, fuel igniting means, means for bringing into operation said fuel feeding means and said fuel igniting means, an electrically operated cut- I out mechanism operable after a timed period to interrupt the operation of said fuel feeding means, means including a switch for deenergizing said cut-out mechanism to render the same inoperative, a relay coil for said switch, an energizing circuit for said relay coil and a condenser connected across said relay coil, said condenser being of sufiicient capacity relative to the resistance of said circuit means to appreciably retard the energization of said relay upon establishment of said energizing circuit by reason of the time required to charge said condenser.

21. In a burner control system, fuel feeding means, fuel igniting means, means 'for bringing into operation said fuel feeding means and said fuel igniting means, an electrically operated cutout mechanism. operable after a timed period to interrupt the operation of said fuel feeding means,

means including a switch for deenergizing said cut-out mechanism to render the same inoperative, a relay coil for said switch, a source of electrical energy, circuit means connecting said source of electrical energy to said relay coil, said circuit means including a grid controlled electric valve for controlling the flow of electrical energy from said source to said relay, a condenser, and connections between said condenser and said grid effective to retard the flow of current to said relay coil until said condenser has been charged.

22. In a burner control system, a pilot burner, a main burner, an electrically operated device associated with each burner for controlling the flow of fuel to said burner, switching means for effecting the energization of the pilot burner fuel flow controlling device, means responsive to the pilot burner flame and operative upon the same assuming an igniting condition to cause the energization of the main burner fuel fiow controlling device, further switching means, said first named switching means being operative only after an initial predetermined operation of said further switching means including the movement of said switching means between two of its circuit controlling positions, and means for preventing the completion of said initial predetermined operation of said further switching means in the event of a condition simulating that of the pilot flame when in igniting condition.

23. In a burner control system, a pilot burner, a main burner, an electrically operated device associated with each burner for controlling the flow of fuel to said burner, first and second relays each including a relay coil and normally open switching means which is closed upon energization of the relay coil, an energizing circuit for the pilot burner fuel flow controlling device controlled by the switching means of said second relay, an energizing circuit for said main burner fuel flow controlling means controlled in part by the switching means of said first relay, means for energizing said second relay coil to establish the circuit through said pilot burner fuel controlling device, means responsive to the pilot burner flame and operative upon the same assuming an igniting condition to energize said first relay coil to complete the circuit to said main burner fuel flow controlling device, and further means to energize and deenergize successively said first relay coil prior to the energization of said second relay in order initially to test the operation of the system.

24. In a burner control system, a pilot burner, a main burner, an electrically operated device associated with each burner for controlling the flow of fuel to said burner, switching mechanism for efiecting the energization of the pilot burner fuel flow controlling device, means responsive to the resistance of a path including a gap bridged by the pilot flame, said means being operative upon the resistance of said path subsequent to the energization of said pilot burner fuel controlling device assuming a value accompanying the bridging of said gap by a flame to cause the energization of said main burner fuel flow controlling device, said last named means being operative upon the resistance of said path prior to the energization of said pilot burner fuel con trolling device assuming a value lower than that normally existing when the gap is open to prevent said energization of the pilot burner fuel flow controlling device, and means to decreasethe sensitivity of said resistance responsive means upon the energization of said pilot burner fuel flow controlling device so as to necessitate said path having a lower resistance for operation of said resistance responsive means.

WILLIS H. em. 

